Cytotoxic CD4 Th1 cells are emerging as a physiologically relevant and therapeutically useful T cell lineage that can successfully target tumors. Until now the cellular and molecular pathways that program their differentiation have been poorly understood. Our preliminary data demonstrates that CD134 (OX40) costimulation induces nave self-reactive CD4 T cells to differentiate into cytotoxic Th1 effectors, and the addition of CD137 (4-1BB) costimulation maximizes their clonal expansion. This differentiation process requires IL-2, and the T-box transcription factor Eomesodermin (Eomes). The experimental plan will test if Eomes programs bone fide memory potential. If so, this will help to resolve the recurring controversy regarding the existence and basis of CD4 T cell memory. Perhaps our most unexpected observation is that CD134 plus CD137 dual costimulation-programmed antigen-specific cytotoxic CD4 Th1 cells imprint antigen - inexperienced bystanding T cells with similar functional capacities. We will test if these polyclonal bystander - activated effectors can be exploited to target antigen-loss variant tumor cells that have down-regulated epitopes recognized by specific cytotoxic T cells elicited by immunotherapy, and if these bystander T cells mediate pathogenic side effects associated with immunotherapy. In summary, the proposed experiments will mechanistically dissect how CD134 plus CD137 dual costimulation induces specific and bystander cytotoxic CD4 Th1 cells, and develop therapeutic strategies to harness their potential to control tumor growth that can be translated clinically to treat human cancer patients.